The invention relates to a method of exposing light-sensitive materials as stated in the introductory portion of claim 1, and to an apparatus for performing the method as stated in the introductory portion of claim 13.
EP 0 589 652 A describes a so-called single raster scanning system (ROS) having a rotating mirror in which a rotating polygonal mirror reflects a xe2x80x9cbundlexe2x80x9d of laser beams with different wavelengths. The laser beams are very close to each other (10 xcexcm), but still spatially separated perpendicularly to the axis of rotation of the polygon (fast scan direction), and are split by means of optical filters to optical paths to the associated photo-receptors.
This spatial separation, however, is problemetic in several different connections, since the mechanical structure of the optical system will invariably have an increased complexity, and at worst involves a deterioration of precision and possible exposure resolution.
U.S. Pat. No. 4,962,312 discloses a multi-station printer in which a plurality of light beams with different wavelengths is gathered to a beam by means of a beam splitter and is then conducted to the stations of the printer via a polygon, in dependence on the wavelengths of the light beams.
A serious drawback of this system, however, is that it may be difficult to achieve the necessary coincident optical axes by using beam splitters per se. This has the additional consequence that the system gets more expensive and complex if a reasonable result should be desired.
A further drawback of this structure is that the light beams may be affected critically by temperature gradients, turbulence, etc.
When at least one additional light source generates in additional light beam with a wavelength xcexx which is different from xcex1, said additional light beam with the wavelength xcexx being conducted via the input end of the optical system to the output end of the optical system so as to be geometrically coincident with the first light beam at the output end of the optical system, said additional light beam being conducted from there to an additional exposure point on the light-sensitive material in dependence on xcexx or the mutual difference between the wavelengths xcex94xcex, said light being conducted completely or partly from the input end of the optical system via one or more light guides, preferably optical fibres, to the output end of the optical system, it is possible to optimize the exposure rate, as the necessary mechanical movement between output end and exposure point may be reduced to a minimum. It should moreover be noted that the method may be adapted to the nature and the rate of the mutual movement in a simple manner, thereby allowing the method to be preferred on the same apparatus with a different resolution, as the distance between the first exposure point and the additional exposure point or points may be adjusted e.g. by changing the wavelengths of the light sources. The method is thus extremely flexible and may be adapted for many applications without complicated mechanical or electronic measures.
The invention is thus unique in that light with different wavelengths may be missed in an optimum manner in a light guide, or e.g. an optical fibre, to a geometrically coincident light beam, thereby making the subsequent separation simpler.
The use of an output end in the form of a light guide, e.g. an optical fibre, moreover allows simpler adjustment of the output end relative to the rotating optical element, as, physically, a fibre end is much simpler to handle and to position than a beam splitter per se.
Further, it is also possible to exchange the individual laser sources in the complete application separately, without this requiring a new overall calibration of the optical system.
An additional advantage of using light guides or optical fibre means between laser sources and the output end of the optical system is that it is simpler to position laser sources and optional associated modulators in primary consideration of where the mechanical position is most advantageous for the apparatus concerned. This involves a special advantage for general purpose printers or scanners.
It has also been found that this advantage is particularly pronounced for e.g. multi-station printers as it is frequently desired, for cost-saving reasons, to expose different physically separated exposure points, e.g. one exposure point per colour, with just one rotating optical element, for which reason a geometrical separation of the light beams results in a greater mechanical and optical complexity.
It will likewise be appreciated that, according to the invention, the precision may be maintained even at very high speeds of rotation.
Thus by mixing the various light beams with correspondingly different wavelengths so that these are fed geometrically coincidently from the output end of the optical system, it is possible to carry out a relatively uniform and simple optical control via the rotating reflector to the exposure face.
Thus, by avoiding a spatial separation between the beams at the output end of the optical system it is possible, in a simple manner, to avoid the complicated geometrical relations that exist between the output end and the axially rotating reflector or mirror, thereby avoiding e.g. twisting on the exposure face in a simple manner.
The invention thus allows all light beams to be centered on the rotating reflector, thereby obtaining rotary symmetry for the beams which are subsequently reflected by the exposure face, irrespective of the position of the reflector or the mirror in the path of rotation.
From two to several light sources may be used according to the invention.
The light sources used may be both of the same type or of different types.
The optical system comprises the necessary optical features to conduct light from source to exposure point. If the light sources are not modulated directly, the optical system may also comprise optical modulators.
According to the invention, a reflector is taken to mean e.g. a mirror, a pentaprism, or optical elements having a sufficiently deflecting effect.
Further, it is possible to conduct the light to the exposure points via a compact optical arrangement, as the optical system is hereby well-defined and easy to calibrate.
The use of optical fibres in this connection provides an additional advantage, as the spatial distribution of the complete light beam is improved considerably, since the light from the various light sources is coincident if the light from all light sources is conducted precisely to one optical fibre.
An additional advantage obtained by using optical fibres in this connection is that the precision is improved considerably, considering that the optical reflector performs very fast revolutions with consequent turbulent flows around the rotating optical element of the scanner. The use of optical fibres minimizes the optical temperature-sensitive distance, which is particularly imporatant when using multi-beams whose mutual focusing distance on the exposure face is affected critically by temperature gradients.
Moreover, it is possible to arrange heat-generating components suitably relative to e.g. heat-sensitive components. The light source may thus be positioned freely as desired.
When the light is conducted from the individual light sources to a common optical fibre in the optical system via one or more couplers, a particularly advantageous structure of the optical system may be obtained, as an optical coupler causes relatively low optical losses in the mixing of two or more light waves from two or more optical fibres to one optical fibre.
Thus, in several applications, an optical T-coupler will exhibit very low optical losses in the actual mixing.
The optical coupler is particularly advantageous in connection with e.g. general purpose printers, as optical mixing in a coupler is relatively simple and inexpensive, just as it is possible to exchange laser sources separately and to adjust these to the individual input end, without this necessarily calling for overall optical adjustment of the system.
It should be mentioned that within the scope of the invention there are other possibilities of geometrically mixing light beams having several different wavelengths to a combined light beam which contains all these wavelengths.
For example, it may be mentioned that a beam splitter may advantageously be used in several application types using linearly polarized light, as, in many cases, a beam splitter can better maintain the polarization than an optical coupler.
When the light is conducted from the individual light source to an exposure point via the optical system, an optical element with light-splitting properties reflecting or transmitting the light to the exposure point in dependence on the wavelength of the light, an advantageous and preferred embodiment of the invention is obtained, as the combined light signal may be split in a simpler manner into a plurality of differently oriented modulated light signals corresponding to the plurality of light sources with different wavelengths, thereby providing a corresponding plurality of exposure points on the light-sensitive material. The optical element used may e.g. be produced holographically or lithographically according to the intended purpose.
The optical element may thus be a mirror having an integrated optical grating or a grating which transmits the incident light to the exposure points.
It is thus possible to increase the exposure rate in internal drum scanners considerably. As the requirements with respect to the structure of the inner mechanical parts and the system as a whole are very critical for internal drum scanners, the invention is particularly advantageous in this connection, since the invention thus enables a very compact structure of the carriage of the scanner.
When the light beams are separated by means of a grating arranged at the output end of the optical system, thereby making the angle of incidence of the light to the rotating optical reflector and thus the exposure point dependent on the wavelength of the incident light, an alternatively embodiment of the invention is obtained, said embodiment being less exacting with respect to the dimensioning of the optical element.
The use of a transmitting grating in this manner also allows the grating to be oriented perpendicularly to the incident light, which inter alia provides advantages in terms of dimensioning.
When the light is conducted from the individual light source via the optical system to a rotating reflector having a plurality of filter layers, said light being either transmitted to the next filter layer on the reflector or reflected toward an exposure point in dependence on the wavelength, a further and efficient embodiment of the invention is achieved, it being possible to provide the reflectors with a plurality of coating layers having given filter and reflection characteristics.
In this connection an advantage is also obtained with respect to the localization of the exposure points, as minor inaccuracies in the wavelengths of the light sources do not displace the exposure point. This provides advantages in connection with light source dimensioning and light source dimensioning requirements.
The method allows a very well-defined orientation of the split modulated light toward the exposure points, as the split light is conducted in parallel from the reflector to the exposure points at a mutual distance. The distance between the exposure points may hereby be dimensioned uniquely and be determined on the basis of the thickness of the individual filter layers.
This, in principle, makes the localization of the exposure point independent of the distance between reflector and exposure point, thereby facilitating the dimensioning of e.g. focusing optics.
When the light is focused toward the exposure points by means of a focus lens integrated in the optical element, it is possible to focus the modulated light on the light-sensitive material.
When the wavelength of the individual laser light source is measured currently and the wavelength of the laser source is adjusted in dependence on this measurement signal on the basis of a reference wavelength xcexref given or desired for the laser light source, an effective control, necessary in several cases, of the wavelength of the light sources is obtained, as even relatively small deviations with respect to the reference wavelength may result in an undesired deviation in the localization of the exposure points.
When the light is modulated in the optical system, it is possible to control the wavelength of the individual light sources in an effective manner, as a non-modulated light source, in the form of e.g. a laser diode, can be maintained more easily on the desired wavelength in the event that the light is modulated outside the laser diode.
When the light is conducted from the individual light source to an exposure point via the optical system, a rotating optical element reflecting or transmitting the light to the exposure point, the light path between light source and exposure point extending through at least one optical element placed in the light path and having focus-changing properties, it is possible to perform splitting or compensation for e.g. the direction and the focusing of the light, which may e.g. be successive.
When an optical compensating element is used, it is also possible to compensate completely or partly for variations in the illumination wavelengths caused by inaccuracies in the illumination sources. Thus, the compensating optical element makes it possible to maintain the focus and the illumination direction geometrically before and/or after splitting of the light, and thereby the mutual distance between the exposure points.
When the light is conducted to the exposure points via at least one compensating optical element with focus-changing properties positioned between light sources and exposure points, said compensating optical element rotating with the rotating optical element, it is possible to obtain a precise focusing of all exposures points on the film, as the compensating optical element compensates for the mutual change in the focusing between the exposure points which is provided by the splitting of the light.
When the compensating optical element rotates with the optical reflecting element, it is ensured that the exposure points do not get twisted, and the mutual position is maintained on the film.
When the light is conducted to the exposure point via at least one optical element with refractive properties positioned between the rotating optical element and the exposure point, said optical element with refractive properties rotating with the rotating optical element, an advantageous embodiment of the invention is achieved.
When the light is conducted from the individual light sources to a common optical fibre in the optical system via a beam splitter, a particularly advantageous embodiment of the invention is achieved, which is particularly pronounced when it is desired to maintain the linear polarized light, as a beam splitter advantageously maintains the polarization during this mixing.
When the apparatus comprises at least one optical diffractive element arranged to split incident light in dependence on the wavelength of the light, the optical diffractive element being adapted to rotate at the same speed of rotation as the rotating optical element to maintain a fixed mutual position between the optical rotating element and the optical diffractive element or elements, it is possible to perform multi-beam illumination with rotation mechanisms or compensation optics or mechanisms.
When the optical diffractive element or elements are formed by optical gratings, a simple embodiment of the invention is achieved.
When the output end of the optical system is formed by a light guide, a particularly advantageous embodiment of the invention is achieved, as the light guide can xe2x80x9ccontainxe2x80x9d or, more particularly, conduct light beams with different wavelengths and modulation in a simple manner in one geometrically coincident transmission.
Light guide is preferably taken to mean an optical fibre, a self guide or the like.
When at least one optical compensation element is arranged between the output end of the optical system and at least one exposure point, so that all light beams with different wavelengths are focused mutually spaced on the exposure face, a special and important embodiment of the invention is achieved, as, according to the invention, said splitting may have as a result that the focus of the individual light beams is not sufficiently close to or coincident with the exposure face, unless simple focus compensation is performed for the light beams concerned.
It should be stressed that the above-mentioned embodiment may also be implemented as a pre-focusing of the light beams, so that these are suitably conducted to the reflector and from there for focusing on the exposure plane.